The PX4 is one of the classic audio power triode valves. Here are a pair of PX4s with bright anodes in the classic shouldered style of envelope. Early PX4s (scroll down) had a balloon envelope and the later production PX4s had uprated specifications and a blackened anode to dissipate the extra heat produced.
The filament is a single wire formed vertically within the anode cavity into four inverted V's. The grid is a single wire formed between two vertical notched rods 1.5 mm in diameter. The grid zig-zags around the filament.
When used in class AB1 push-pull a pair of PX4's would deliver 13 Watts.
Oblique view of the anode which is much wider than the front to back dimension. The gettering is to one side only.
The dome showing the four filament springs on a second top mica.
The classic envelope is 60 mm in diameter and, excluding the B4 base pins, is 123 mm tall.
References: Datasheet, 1005, 1040 & 1043. Type PX4 was first introduced in 1929.